1. Field of the Invention
The present invention relates to an electric tool with operation modes for providing different outputs to an object, and particularly the electric tool for applying a rotational force to the object such as bolts, nuts and screws through an output shaft driven by a reversible motor.
2. Disclosure of the Prior Art
To carry out operations of tightening a fastening member such as a bolt, nut or a screw and loosening the fastening member, electric tools have been widely used. For example, an impact rotary driver is disclosed in Japanese Patent Early Publication No. 7-314342. According to this tool, when an output shaft is rotated in a forward direction by a reversible motor, the operation of tightening the fastening member can be performed. On the other hand, when the output shaft is rotated in the reverse direction, the operation of loosening the fastening member can be performed. In addition, the tool has the capability of intermittently providing an impact force to the fastening member at the finish of the tightening operation or at the start of the loosening operation. Therefore, it brings improvements in reliability and easiness of the tightening and loosening operations.
By the way, allowing this kind of tool to be available to the fastening members having different sizes improves working efficiency. For example, if the output shaft can be driven by a suitable one selected from a plurality of operation modes with different torques, it becomes possible to apply an appropriate rotational force to the respective fastening member without causing damage to the fastening member. However, when it is needed to alternately perform the tightening operation of a relatively small-sized bolt and the loosening operation of a relatively-large-sized bolt, the suitable operation mode must be reset every time that the rotational direction of the output shaft is switched. Consequently, it will lead to a considerable decrease in working efficiency. In addition, there is a fear of deteriorating work safety at high places.